Age-dependent immune dysfunction, immunosenescence, may contribute to the increased incidence of cancer and other age-related diseases in old people. It is believed that many age-related diseases are caused by accumulation of oxidative damage from oxygen free radicals. In particular, the mitochondrion, the major site of free radical generation in the cell, is susceptible to oxidative damage to its' DNA as well as proteins and lipids, and mitochondrial dysfunction is thus believed to play a major role in aging, and-age-related diseases. Immunosenescence have been studied extensively in splenic T cells from old mice, in which, the response to stimuli is greatly attenuated. This is, partially, the result of inhibition early signaling events (e.g the rise in cytoplasmic free Ca2+, protein phosphorylation) but also several additional defects. We suggest that some of these defects are the result of mitochondrial dysfunction. Thus, the inhibition of Ca2+ signaling may be due to modulation of mitochondrial calcium metabolism while the inhibition of protein phosphorylation may result from inhibition of ATP synthesis. The age-dependent alterations in lymphocytes subsets may also result from mitochondrial-dependent aberrations of programmed cell death. To test the hypothesis we shall study splenic lymphocytes from young and old mice and characterize the activities of the systems of oxidative phosphorylation (e.g. respiratory complexes, ATP synthase), the phosphate potential, pH1, mitochondrial membrane potential and mitochondrial calcium metabolism uptake and efflux systems, the megachannel). These studies will progress from whole cells to saponinpermeabilized cells and isolated mitochondria. Initially these experiments will be carried-out with unfractionated lymphocyte populations and later selective experiments will be carried-out with lymphocyte subsets (e.g. B and T cells, naive and memory T cells, cytotoxic T cells) and also with other cells of the immune system (e.g. macrophages and NK cells). These experiments should elucidate the role of mitochondrial dysfunction in immunosenescence and may lead to new strategies in prevention of cancer and other age-related diseases.